2. I've got 2 grids from the same controller (an own controller). If i comment one of them, the other one works fine, but when i use both, it just works the second one. When i select an item from the first grid, it selects the second one.

Do you know why is it? i tried to render my grids in different panels but it didn't work...

1. you need to use a renderer. I think there was a feature request to make that a little easier, it was posted in last 3 months or so. It was just a request, I don't recall if sample code was posted. Short answer is use a renderer. You may also want to check the hiddenName config if you plan to submit to backend.
2. smells like you've re-used an id or store. You didn't post any code, so who knows?